No, they were not at all badly portrayed in the movie...
The first Indian: Undoubtedly a hardened criminal wholly deserving of his fate--he most likely murdered someone while in a drunken state, hence his punishment. This whale-like tub of shit was as close to reality as they could get.
The second two Indians at the outpost: The Indian who was kicked by Rooster was some kind of a degenerate, malignant dwarf (and certainly not a child) who was guilty of aiding and abetting Tom Chaney--note the shit-eating grin on his face even after he was kicked. Punks like this INFEST Indian Country and are usually involved in all kinds of unsavory activities.
The older Indian on horseback: Outrageously funny that he'd trade a corpse for something. Rooster gave this man the ultimate in respect by giving him the body in exchange for being on his land and for a little information.
For more on the subject, see Hanged Indian in True Grit and The Best Indian Movies.